1. Field of the Invention
This invention relates to an automated sample conditioning module for condensing, cooling and reducing pressure of hot fluid samples. In particular, the invention relates to a system for conditioning steam and hot water samples at a constant sample flow rate and velocity, automatically adjustable at all varying source pressures, for preparing the sample for analysis.
2. Background of the Related Technology
An operator of a steam and hot water system at a utility power plant or other steam plant must carefully monitor and control water chemistry in the system. Common water impurities may cause corrosion, restricted water flow, fouling of heat exchange surfaces, mineral deposits and build-up of crud or scale, and a variety of other operating problems in a power plant. Successful operation of a power plant therefore requires frequent collection of steam and hot water samples which can be analyzed to provide useful information on the chemistry of the steam-water cycle.
A sample from a steam or water system must be carefully handled in order to assure that the sample is representative of conditions in the system. Analysis equipment is typically capable of measuring very small concentrations of contaminants in water, some equipment being capable of measuring in the parts per billion (ppb) range. On-line water chemistry instrumentation often includes analysis for pH, conductivity, dissolved oxygen, hydrazine, sodium, and chloride content. Unless the sample is representative of current conditions at the source, even the most sensitive analytical equipment is useless. Alteration of the properties of the sample during the sampling process can result from a number of causes, such as non-representative sample extraction, deposition or re-entrainment of solids, absorption or release of impurities, air leakage into the sample line, or contamination of the sample during handling.
This invention is concerned primarily with conditioning of the sample, namely, condensing, cooling, reducing pressure and controlling flow rate of the sample to prepare it for analysis.
Recently the need for continuous measurement of water chemistry parameters has been recognized in the power plant industry, which in turn requires the continuous extraction of a sample for analysis. The Electric Power Research Institute (EPRI) and several plant chemistry experts recommend a constant liquid sample velocity of about 6 feet per second at all varying source pressures. A study by EPRI showed that for most liquid sample analysis situations sampling at a linear velocity of 6 feet per second decreased the sample line loss of insoluables to a negligible amount, and it minimized the time required to establish equilibrium between the amount of particulate transferred to and from the sample line. (See Guideline Manual on Instrumentation and Control for Fossil Plant Cycle Chemistry, EPRI CS-5164, Project 271, Final Report, April 1987, Chapter 2.)
Sampling from a steam line provides a further challenge because of the complex character of steam. By combining several isokinetic sampling nozzles in a primary steam line, one can obtain desirable sample line gas and liquid velocities. Because the sample line usually contains two-phase flow, due to condensing of the sample, the liquid and gas velocities constantly change along the sample line length, although the mass flow rate stays constant.
Steam and water samples taken from a power plant system are still quite hot and at high pressure, much too hot and at much too high a pressure to admit to most analysis instrumentation. Therefore, it is necessary to condense and cool a steam sample, and to cool a water sample, and then to reduce the pressure of the sample to near ambient level so the sample can be introduced to instrumentation at suitable conditions for analysis. Condensing and cooling is necessary before pressure reduction because steam would super heat and water would flash if the pressure was reduced before condensing and cooling.
One known system for conditioning a sample is the applicant's manually controlled, single-line sample panel for conditioning steam and water samples. This manually operated device includes a cooler for condensing and cooling the sample, and a pressure reducer comprising either a needle valve or applicant's variable pressure reducing element (VREL.RTM.). The VREL, used to reduce the pressure and to control the flow of a high pressure liquid sample, is a rod-in-tube device. The pressure of the incoming sample is reduced as the liquid is forced to travel through the narrow annular gap between a stepped rod and the inner diameter of the tube. The pressure drop through the VREL is a function of the length of the rod which is inserted into the tube. The flow through the VREL can be adjusted by manually changing the position of the rod in the tube. The manually operated sample panel provides for sample outlet to either analyzing instruments or for a grab sample or both.
The primary drawback of applicant's single line sample conditioning panel is that it must be manually operated and controlled. The steam cycle in a power plant is very dynamic, especially during start-up, so the device must be periodically, and at times continuously monitored and adjusted by an operator. Also, crud build-up in the sample line, cooler and VREL requires periodic manual adjustment of the VREL and cooling water flow.
U.S. Pat. No. 4,978,506 to Calderwood discloses a system for monitoring corrosive elements in the primary and secondary fluid systems of a nuclear power plant. The system includes elements for conditioning the sample for analysis. In the system disclosed by Calderwood, a sample is removed from the steam system by an isokinetic nozzle (reference number 11) connected to a sample line (12) and an isolation valve (14). The sample pressure is reduced by a capillary pressure reducer (16). A cooler (18) reduces temperature of the sample from an operating range of about 500.degree. F. to 600.degree. F. down to about 100.degree. F. Indicators are included in the sample line (12) for pressure (20), temperature (22), and flow rate (24). A throttle valve (26) controls sample flow rate.
The capillary tubing mentioned in Calderwood may provide good service provided that the source pressure doesn't change and that the tubing doesn't plug. One of the drawbacks of using capillary tubing to reduce pressure is that sizing of the tubes must be done by trial and error, and once the length is set, the flow rate is not adjustable. Since the capillary tube is a fixed element, any change in source pressure causes a change in pressure drop and flow rate. At start up and shut down, in particular, the flow rate at lower pressures will be insufficient to provide a contemporary sample. Probably the greatest difficulty with capillaries is that their small bore is easily plugged with a piece of scale or crud. Once plugged, the capillaries cannot be easily unplugged and the line must be shut down while the tubing is cleared or replaced.
The Calderwood patent further mentions, at column 8, lines 51-57, that sample runs can be controlled automatically by a programmable controller to, for example, set the throttle valve (26) to ensure a constant sample flow rate. However, neither the programmable controller nor a program logic are shown in the drawings or described in the specification. Also, due to fouling of the capillary tubing, the mere addition of a programmable controller to the Calderwood system would not ensure constant sample flow rate. The application of an automatic controller to a sample conditioning system is just not as easy or economical as Calderwood suggests, and, more importantly, such an automatic system is not disclosed in the Calderwood patent.
Other systems for sampling, monitoring or analyzing a power plant steam-water cycle are disclosed in U.S. Pat. No. 4,766,550 to Byers et al., U.S. Pat. No. 4,713,772 to Carlson, U.S. Pat. No. 4,472,354 to Passel et al., and U.S. Pat. Nos. 3,880,226 and 3,871,444 to Houser et al. As with the Calderwood patent, these other patents do not disclose a specific device or system for automatically condensing, cooling and de-pressurizing steam and water samples at a constant flow rate.